The Hidden Life of Algae Underground

The term “algae” designates a most diverse and ancient group of organisms that is polyphyletic by evolution and artificial by taxonomy. Its only common feature is the ability to perform aerobic photosynthesis. Algae range by size from tiny cyanobacterial cells of the picoplankton to the giant kelps dominating rocky coastlines. They settle most diverse aquatic habitats such as hot springs and Arctic ice, live on and in rocks and various organisms, travel by air currents for thousands of miles and can be found in groundwater. Algae have taken an important part in the evolution of Gaia and gave rise to embryophytes (plants: bryophytes and spermatophytes).

In most textbooks, it is common wisdom that the success of algae is dependent on sufficient light available for net photosynthesis. However, there exists another world, hidden and without light, offering a plethora of aphotic habitats. That is the world underground, which is not only settled by bacteria, fungi and the terrestrial fauna, but also by algae.

The following is meant to give an introduction into what we know and suggest on the life of algae underground, that is on algae not living, as crust forming algae do, on but in soil substrates. The leitmotiv shall be to look for lessons they can tell us about the tremendous potential of life to adapt to habitats, that – at first glance – appear to be rather hostile and strange. For practical reasons all kinds of both pro- and eukaryotic algae living on and within soil substrates will be designated as “soil algae.”